Pulse number modulation system



NOV. 4, 1947. H o PETERSON 2,430,139

PULSE NUMBER MODULATION SYSTEM Filed Jan. 8, 1944 2 Sheets-Sheet l IIIIIIIIIIIIII llllIlIii..

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Patented Nov. 4, 1947 PULSE NUMBER MODULATION SYSTEM Harold 0. Peterson, Riverhead, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application January 8, 1944, Serial No. 517,484 9 Claims. (01. 2506) This invention comprises a pulse number modulation system for communication purposes.

A primaryobject of the present invention is to provide a communication system employing spaced pulses of substantially equal length, for communication purposes, and wherein the number of pulses transmitted varies in accordance with the useful modulation.

Another object of the invention is to provide a radio transmitter having a pulse generator controlling a radio frequency oscillator over a path including an electronic valve or shutter arrangement Whose open periods are, in turn, controlled as to duration by the signal modulation potentials.

A further object is to provide a pulse communication' system having a transmitter circuit for modulating the number of pulses transmitted in accordance with the useful modulation, in combination with a receiver equipped with a shutter or valve circuit adapted to pass the incoming c pulses at the maximum pulse frequency.

In accordance with the invention, it is proposed to transmit equal length and constant amplitude pulses of radio frequency energy having a recurrence rate always at least twice as high as the highest modulation frequency. It is preferred that the pulses be short compared to the time interval between them, though this is not essential since the pulses may, if desired, have 50% marking time. The number of pulses transmitted is made to vary in accordance with the modulation potential, or, putting it in other words, si nalling is accomplished by omitting various numbers of pulses. Thus, if the maximum number of pulses transmitted is 36,000 per second, and the average modulation frequency is 3000 cycles per second, then the number of transmitted pulses is made to vary from 36.000 per second to zero in accordance with the useful modulation. In the case of no signal modulation. the number of transmitted pulses would be 18,000 per second or half the maximum available number of pulses. mum modulation in one direction, let us assume the ositive direction, the number of pulses would be 36,000, whereas for the maximum modulation in the other direction, negative in this assumed case, there would be no outgoing pulses. At the receiver there are provided pulse selective circuits and a synchronously controlled electronic gate or shutter arrangement for enabling the pulses to pass through to the acoustic transducer. If the pulses employed for communication are short compared to the time interval between them, then the peak power of the transmitted pulses can be For maxivertical line represents a pulse.

increased by a factor which is proportional to the ratio of the space to the mark time.

Other objects of the invention will appear from a reading of the following description which is accompanied by a drawing, wherein:

Fig. 1 graphically illustrates three conditions for the transmitted pulses in the practice of the invention;

Fig. 2 diagrammatically illustrates a pulse transmitter circuit for producing pulses modulated in number in accordance with the useful modulation;

Fig. 3 graphically illustrates the operation of the transmitter of Fig.2;

Fig. 4 shows circuit details of the transmitter of Fig. 2;

, Fig. 5 diagrammatically illustrates a receiver for use with the transmitter of Fig. 2; and

Fig. 6 graphically illustrates the operation of certain circuits of the receiver of Fig. 5.

In Fig. 1 there are illustrated three pulse modulation conditions A, B and C which can be obtained by means of a transmitter in accordance with the present invention. ,In this system, the pulse length, pulse timing, and pulse amplitude are constant. Line A represents the condition in which the transmitter sends out a maximum number of pulses for a condition of maximum modulation in one direction. It is assumed that condition A represents the condition of maximum modulation in a positive direction, and that the maximum number of pulses is 36,000 per second, or six pulses for each of a second. Each Line B represents a condition of no modulation. It will be noted that in this condition of no modulation only half the number of pulses of line A are transmitted. Thus, in line B only 18,000 pulses per second are transmitted in groups of three. Line C represents the condition of maximum modulation in the opposite or negative direction relative to the direction of modulation represented by line A. In line C there are no pulses transmitted. Thus, the number of pulses sent out by the transmitter varies from 36,000 per second, represented by line A, to zero, represented by line C. For conditions of modulation between the maximum condition and the no modulation condition, there will be transmitted pulses'greater or less than the number illustrated in line B depending upon the degree and direction of modulation. It will thus be seen that signalling is accomplished by omitting various numbers of pulses in groups.

Fig. 2 schematically illustrates one transmitter circuit in accordance with the invention for producing and transmitting pulse signals of the character illustrated in Fig. l. The transmitter of Fig. 2 comprises a rectangular wave pulse generator ill, a pulse keyer or shutter II, and a radio frequency generator I'o. The keyer or shutter Ii serves as a valve to permit a desired number of rectangular wave pulses from the generator ii! to pass through the keyer and control the generation of radio frequency pulses by radio frequency generator I6. Coupled to the keyer II there is a keyer or shutter width control circuit I2, the latter in turn beingcontrolled by voltages from a local modulation circuit-l3 and a saw-tooth generator I4. The localfmod'ulation circuit I3 is shown in Fig. 2 as comprising a microphone and a suitable amplifier I5. The term keyer used herein is generally referred to in the art as a gate or shutter.

are impressed on a suitable transmitting antenna IT.

The operation of the system of Fig. 2 may be better understood from an inspection of the graphs of Fig. 3. The voltage wave forms of lines D, E and F of Fig. 3-appear in the connections of Fig. 2 which are labeledwith the same letters. Line D of Fig. 3 shows a series of pulses of rectangular wave form produced by the pulse generator ID of Fig. 2 and impressed on the keyer or shutter II. The keyer II is normally biased to anode-current cut off condition and is controlled by the keyer width control circuit I2 to permit the passage of a. desired number of pulses of line D- for controlling the generation of radio frequency pulses by oscillator apparatus IS in accordance with the modulation. Line E of Fig. 3 represents the output pulse from the keyer width control I2 which turns on the keyer II to permit the passage through the keyer of the pulses from the pulse generator ID. The bi-directional arrow on the starting edge of the pulse of line E indicates that the starting time of this pulse is variable. The trailing edge of the output pulse of line E from the keyer width control circuit I2 is fixed. The starting time of the output pulse E depends upon the combined voltage from the local modulation circuit I3 and the saw-tooth generator I4. Referring to line G of Fig. 3, which shows the saw-tooth voltage wave obtainable from the saw-tooth generator I4, it will be noted that the starting time of the pulse of line E depends upon the threshold or critical voltage value indicated by the'hori'zontal dash line F which intersects the saw-tooth voltage wave. By adding the modulation potential from the circuit I 3 to the saw-tooth voltage wave, I am able to valy the length of'the pulse E. In the absence of modulation from circuit I3, the output pulse E from the keyer width control circuit I2 will be as shown and have a duration T. By adding positive potential to the saw-tooth wave by means of themodulation circuit I3, the threshold value F of the saw-tooth wave necessary to open the keyer width control I2 will be moved to the left or down on the saw-tooth slope for widening the length of the pulse E. By a ding negative potential to the saw-tooth wave by means of the modulation circuit I3, the threshold value of the saw-tooth voltage wave will be moved to the right or upon the slope of the saw-tooth wave, thus narrowing the'width of pulse El. Alternatively, the wave shape G of Fig. 3 may be sloping on both sides, in which case the timing of both ends of the pulse E would be varied by the signal modulation.

Pulses of radio frequency energy derived from the oscillator I6 Fig. 4 shows the circuit details of the keyer II, the keyer width control circuit I2, and the manner in which the modulation potential is combined with the saw-tooth voltage wave to control the keyer. In Fig. 4 the keyer II is shown as a pentode vacuum tube to one of whose signal grids there is applied the output from the pulse generator I0 and to whose. other signal grid is coupled the keyer width control vacuum tube i2. The input circuit of vacuum tube I2 is shown coupled in series to both the output of saw-tooth generator I4 and the output of amplifier I5.of. the local. modulation circuit through tapson variable potentiometers I l and I5. The saw-tooth generator I4 is arranged to generate a saw-tooth voltage wave in the negative direction so as to swing or vary the grid bias on the tube l2 more negatively, in order to cut-off the flow of current through tube I2. The control grid of tube I 2 is normally biased positively relative to its cathode by means of battery BAT. Normally, with no saw-tooth voltagewave from the generator I4 and with no modulation from the amplifier 55, the vacuum tube I2 passes current and will produce a voltage drop in resistor R in its output circuit. Since this resistor R is also in the control grid circuit of the keyer I I, it will be evident that when tube I2 passes current, the keyer 2! will be biased tocut-ofi'. In this condition, the rectangular wave pulses from the pulse generator ID will not be able to pass through the keyer I I to control the generation of radio frequency pulses'from oscillator apparatus It. When the saw-tooth generator I4 supplies a negative saw-tooth voltage wave to the grid I2, it will cut'oiT the flow of current in the tube I2 at a particular threshold value. The cessation of current how in tube I2 will cause the voltage across resistor R to rise, as a result of which the cut-off bias on the control grid of the keyer tube II is removed, thus permitting the passage of rectangular wave pulses from the pulse generator I6 through the keyer to the radio frequency generator I6. The threshold value at which the keyer I2 cuts off is so arranged that tube 52 will be cut oil? for one-half the time of operation when there is no" signal. the pulse generator I0 generates 36,000 rectangular wave pulses per second, the keyer. width control circuit I2 will control the keyer II for one-half the time of operation so as to permit one-half this number of'pulses (or 18,000 pulses per second) to pass through the keyer II to control the radial frequency generator IS. The pulses passed by'keyer' II will" be in groups, as illustrated previously in connection with Fig. 1. By means of the local modulation circuit, the output potential from the modulator is combined with the saw-tooth voltage wave to vary the timing of the point on the saw-tooth wave at which the cut-off potential for the tube I2 is reached, thus varying the number of pulses passed by the keyer II in accordance with the signal modulation.

Fig. 5 shows one form of receiver which may be employed in the pulse number modulation system of the invention. The receiver includes an antenna- 40 which is indicative of any suitable energy collectingv device. (and which may be directive or otherwise), a heterodyne detector or a frequency converter 42 which may, if desired, be preceded by a radio frequency amplifier 4| andwhich is fed with energy from. the antenna and also with oscillations from a local heterodyne oscillator 43, and an intermediate frequency amplifier and se- Thus, if

lective circuit 44 in the output of the converter 42. The intermediate frequency amplifier 44 has its output connected to a rectifier 45, in turn followed by a clipper amplifier 46. The clipper 46 removes a portion of the rectified pulse between the top and bottom to thereby provide a clean pulse having the noise ripples removed from the spaces between incoming pulses and from the flat tops of the incoming pulses. It is preferred, as will appear hereinafter, that the clipper be a top and bottom limiter which removes noise and variations above and below certain limits. The clipped pulses in the output of clipper 46 will, however, have variations of their length and phase caused by noise. The middle of these clipped pulses will be constant in amplitude. The clipper 46 is followed by a keyer or gate 41, in turn followed by a low pass filter 48, an audio amplifier 49, and an acoustic transducer 50, the latter of which may be either a loudspeaker, headphones, or any suitable recording device. The low pass filter 48 is arranged to have a cut-off point at the maximum modulation frequency so as to prevent higher frequency components from passing therethrough. In order to synchronize the keyer or gate 41, there are provided a pulse generator 5|, a pulse frequency oscillator 52, and an automatic pulse frequency control circuit 53. The pulse frequency oscillator 52 may, if desired, be a saw-tooth oscillator which operates at the pulse frequency. This saw-tooth oscillator is controlled by the automatic pulse frequency control circuit 53, in turn receiving the incoming pulses from the output of the clipper 46. The synchronization apparatus 53 may be of any well known form, uch as that used in the television art.

In the operation of the receiver, the incoming pulses, after rectification by rectifier 45, may have the appearance shown in line M of Fig. 6. It will be noted that the pulses of line M have numerous ripples which are representative of noise and other variations. These pulses, after being clipped by the clipper 46, will have the appearance of the pulses shown in line N of Fig. 6. The pulses of line N, it should be noted, have the ripples removed and are representative of a desired portion or slice of the pulses of line M. The pulses of line N are of constant amplitude but are variable in width or length, due to the effect of noise. The keyer or gate 41 eliminates the variations in width of the pulses of line N by permitting the passage of only a central portion of each of the clipped pulses, thus providing output pulses from the keyer which are of constant width and constant amplitude. Line P of Fig. 6 indicates the shape of the pulses after passing through the keyer 41. This portion of the receiver for removing noise from the clipped incoming pulses is described and claimed in the copending application of J. B. Atwood, Serial No. 524,548, filed March 1, 1944.

The keyer 41 functions at the maximum pulse frequency which, in this case, is assumed to be 36,000 per second, so that if an incoming pulse is received it passes through the keyer 41. The incoming pulse controls the exact time of initiation of the cycle produced at the oscillator 52. If no pulse has been received, then although keyer 41 is open at the proper time to permit the passage of the pulse, nothing will pass through the keyer because of the fact that the noise ripples have been removed ahead of the keyer, The keyer 41 thus operates to pass only the middle portion of the clipped pulses to thereby eliminate variations 6 in pulse width due to noise. The pulse frequency oscillator 52 is synchronously controlled by the incoming pulses by means of the apparatus 53 to open and shut the keyer at the proper time.

What is claimed is:

l. A transmitter comprising a generator of direct current pulses, a high frequency generator for producing pulses of high frequency energy under control of said first generator, a keyer located between said two generators for controlling the passage of the direct current pulses to the high frequency generator, a keyer control circuit for controlling said keyer, a saw-tooth generator, and a modulator, and connections for combining the outputs of said modulator and sawtooth generator and impressing the combined outputs upon said keyer control circuit.

2. A transmitter comprising a rectangular wave pulse generator and a radio frequency generator, a circuit for impressing the output of said pulse generator upon said radio frequency generator to thereby produce pulses of radio frequency energy, said circuit including an electron discharge device keyer having its control grid biased through a resistor, a keyer control circuit comprising an electron discharge device having a control grid and an anode, said resistor being in the anode circuit of said last electron discharge device, means for supplying the control grid of said keyer control circuit with both saw-tooth voltage waves and modulation potentials, said keyer control circuit being so biased that in the absence of sawtooth voltage waves and modulation potentials the keyer control circuit is conductive and biases said keyer to cut-off by virtue of the flow of current through said resistor, said saw-tooth voltage Waves being in a negative direction to bias said keyer control circuit to cut-off at a particular threshold value during the saw-tooth voltage cycle, to thereby remove the cut-off bias on said keyer and permit the passage of said rectangular wave pulses to said radio frequency generator, said modulation potentials being .additively combined to said saw-tooth voltage waves to move the position of said threshold value on said saw-tooth voltage wave.

3. In a radio transmitter for communication by pulses, a radio frequency oscillator normally arranged to be inoperative, an antenna coupled to the output of said oscillator, means for supplying said oscillator with recurring pulses of direct current energy of such polarity and magnitude as to cause said oscillator to become recurringly operative to produce pulses of radio frequency energy, and means for controlling the number of said direct current pulses which are effective to operate said oscillator in accordance with signal modulation.

4. A pulse communication system having, in combination, a transmitter including a radio frequency oscillator normally arranged so as to be non-oscillating, an antenna coupled to the output of said oscillator, means for supplying said oscillator with recurring pulses of direct current energy of such polarity and magnitude as to cause said oscillator to oscillate recurringly to thereby produce pulses of radio frequency energy, and means for controlling the number of said direct current pulses which are effective to operate said oscillator in accordance with signal modulation, a remotely located receiver having means for rectifying the received pulses, a clipper stage for removing noise ripples from the rectified pulses, a keyer circuit following said clipper, means for rendering said keyer circuit operative in synchronism with the incoming pulses to thereby permit the passage of a desired portion of the rectified and clipped pulses, a filter circuit following said keyer circuit, and an acoustic transducer coupled to said filter circuit.

5. A pulse communication system having, in combination, a transmitter including a radio frequency oscillator normally arranged to be inoper tive, an antenna coupled to the output of said oscillator, means for supplying said oscillator with recurring pulses of direct current energy of such polarity and magnitude as to cause said oscillate; to become recurringly operative to produce ses of radio frequency energy, and means for controlling the number of said direct current pulses which are efiective to operate said oscillator in accordance with signal modulation, a remotely located receiver having a frequency converter circuit for beating down the carrier of the incoming pulses to an intermediate frequency, a rectifier for producing unidirectional pulses from intermediate frequency energy pulses, a clipper stage for removing noise ripples from said unidirectional pulses, a keyer circuit for enabling only a desired portion of said unidirectional clipped pulses to pass therethrough in order to eliminate variations in pulse Width, and means for controlling said keyer circuit to operate at the maximum pulse frequency, a pulse selective circuit following said keyer circuit, an audio frequency amplifier coupled to said pulse selective circuit, and a utilization circuit coupled to said audio frequency amplifier.

6. In a pulse communication transmitter system wherein it is desired to transmit pulses carrying signal modulation components, the method of operation which includes producing constant amplitude and equal duration unidirectional current pulses whose repetition rate is at least twice as high as the modulation frequency, passing said pulses over a path, and interrupting said path in accordance with a characteristic of said signal modulation, to thereby vary the number of said constant amplitude and equal duration pulses which pass over said path in proportion to the instantaneous value of said characteristic.

7. In a pulse communication transmitter system wherein it is desired to transmit pulses carrying signal modulation components, the method of operation which includes producing periodically recurring constant amplitude and equal duration unidirectional current pulses which are spaced from one another and Whose repetition rate is at least twice as high as the modulation frequency, and grouping different numbers of said pulses in proportion to the instantaneous amplitude of the signal modulating wave.

8. In a pulse communication transmitter system having a generator of periodically recurring constant amplitude and equal duration unidirectional current pulses which are spaced from one another, a radio frequency oscillator coupled to said generator over a path, and a signal modulating circuit, the method which includes controlling the operation of said oscillator by said pulses, and interrupting said path at such times and for such intervals in proportion to the instantaneous value of a characteristic of the modulating wave, to thereby vary the number of said unidirectional current pulses which control said oscillator in dependence upon said characteristic.

9. In a pulse communication transmitter system wherein it is desired to transmit pulses carrying signal modulation components, the method of operation which includes producing constant amplitude and equal duration unidirectional current pulses whose repetition rate is superaudible and at least twice as high as the modulation frequency, passing said pulses over a path, and interrupting said path in accordance with a characteristic of said signal modulation, to thereby vary the number of said constant amplitude and equal duration pulses which pass over said path in proportion to the instantaneous value of said characteristic and over substantially the entire pulse repetition range from zero to said superaudible rate.

HAROLD O. PETERSON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,898,477 Bohm Feb. 21, 1933 2,266,401 Reeves Dec. 16, 1941 2,338,512 Harmon Jan, 4, 1944 2,252,293 Ohl Aug. 12, 1941 2,272,070 Reeves II Feb. 3, 1942 FOREIGN PATENTS Number Country Date 541,665 Great Britain Dec. 5, 1941 

